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Application of persulfate for water disinfection purposes

Karapa Alexandra

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Summary

Water is substantial for the preservation of life on our planet. Both ground and surface water (lakes, rivers) are used for consumption by humans. This water needs to pose no threat to human health, to be organoleptically impeccable and completely clean, free of pathogenic microorganisms and pollutants that may impact human wellbeing. One of the most significant issues of our times is the decrease in quantity and the quality degradation of various environmental resources. Nowadays, the ground and surface water resources are being exposed to various types of pollutants (organic, inorganic substances) and microorganisms.Advanced oxidation processes (AOPs) have been recognized as an emerging group of techniques with high oxidation potential and biocidal effect on various microorganisms in aqueous samples. AOPs based on hydroxyl (HO• , E0=1.8-2.7 V) and sulfate radicals (SO4-, E0=2.5-3.1 V) have been widely used as novel oxidants to degrade many pollutants. The formation of highly reactive sulfate radicals may be achieved by peroxydisulfate or persulfate (PS) activation via heat, ultrasound, transition metals, ultraviolet (UV) light, or other means. Enhancing the effectiveness of activated persulfate includes manipulation of traditional methods and development of novel techniques. These processes, being members of the broad category of AOPs, seem to be highly promising as disinfection techniques for the inactivation of various pathogens contained in water.Based on the above, the objectives of this study comprise the following: a) assessment of disinfection efficiency of activated persulfate for the inactivation of fecal bacterial indicators in water and b) investigation of process variables effects and the means of persulfate activation (i.e. UVA irradiation and heat).Experiments were performed using Escherichia coli and Enterococcus faecalis bacteria. This thesis presents experiments carried out in three phases: the microorganisms’ inactivation rates were evaluated using a) UVA/PS, b) heat/PS and c) UVA/PS/heat. What is important to underline is that the experiments were conducted under similar conditions so as to obtain comparable data.Disinfection with UVA/sodium persulfate (15 W) was not particularly effective; there was a 1 Log decrease in bacterial concentration, while when two lamps were used (30 W) there was a 6 Log decrease after 3 h of both bacterial species. When persulfate activation was performed through heat there was a substantial 6 Log decrease after 0.5 h in both bacteria species. Interestingly, Enterococcus faecalis appeared to be more persistent over time and under higher temperatures than Escherichia coli. During the last phase of the experiments, where we applied a combination of the activating parameters mentioned above, a 6 Log decrease of bacterial population was observed .In all cases presented above, it was found that the higher the concentration of sodium persulfate the higher the bacterial decrease. Having compared all the parameters, we can conclude that a combination of the three methods can be named as the fastest and most effective one in activating bacteria with the aid of oxidizing radicals. However, based on contact time, the most effective conditions were temperature of 50 ̊C with sodium persulfate at a concentration of 200mg/L .To sum up, having studied these two bacterial species under the specified disinfection condtions , we can conclude that Gram(-) E. coli proved to be more vulnerable than Gram(+) E. faecalis which possess a thick cell wall.

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